1. Field of the Invention
This invention relates to an apparatus and method for rapid quenching of molten metal. More particularly, it relates to a cooling system for a casting wheel useful in the continuous casting of metallic strip.
For purposes of the present invention, a wheel is a cylinder of substantially circular cross section whose width (in the axial direction) is substantially smaller than its diameter. In contrast, a roller is generally understood to have a greater width than diameter.
Also, for purposes of this invention, a strip is a slender body whose transverse dimensions are much smaller than its length. Strip thus includes wire, ribbon and sheet, of regular or irregular cross section. 2. Background of the Invention
Continuous casting of metal strip can be accomplished by depositing molten metal onto a moving casting wheel. The strip forms as the molten metal stream is attenuated and solidified by the wheel's moving quench surface. For continuous operation, the wheel must be cooled, particularly if it is desired to produce metastable or amorphous metal strip, which requires quenching of certain molten alloys at a cooling rate of at least 104.degree. C. per second, more typically 106.degree. C. per second. Details of a suitable casting procedure have been disclosed in U.S. Pat. No. 4,142,571, and the disclosure of that patent is incorporated herein by reference.
Casting wheels of the prior art generally have been cooled by spraying a fluid, usually water, onto the inner surface of the wheel. Rapid cooling of the quench surface dictates a thin (in the radial direction) wheel supporting a large temperature gradient. However, spray cooling of such a wheel tends to cause thermally-induced distortion or "crowning" of the quench surface, which results in ribbon of nonuniform thickness. For transformer applications, such ribbon, when wound into a core, may have low packing fraction and unsatisfactory magnetic properties.
Another problem with spray cooling is that it generally cannot provide radial-only heat transfer from the outer surface of the wheel to the cooling medium. Lateral (axial) temperature gradients cause nonuniform cooling across the width of the ribbon and lead to undesirably nonuniform strip properties. Finally, cooling efficiency is reduced by the formation of a steam layer, which forms on the inside surface of the wheel and which tends to insulate the surface from the coolant. Higher surface temperature then causes more rapid surface deterioration. Reduced quench rate can cause ribbon of certain glass-forming metal alloys to be undesirably brittle or crystalline, particularly ribbon thicker than about 40 .mu.m.
Rollers used in the manufacture of sheet materials such as glass and linoleum have incorporated longitudinal channels or passages for carrying coolant fluid to prevent temperature gradients which warp the rollers and cause imperfect product. (See, for example, U.S. Pat. Nos. 1,392,626 and 1,781,378). The rollers of those inventions serve to press and form a sheet and play only an incidental role in cooling the product.
Rollers of design similar to those of the aforementioned patents are disclosed in U.S. Pat. No. 3,888,300. These rollers form part of an apparatus for vacuum casting of metals and alloys. The rollers form and guide high-temperature metal ingots as they pass between the rollers. The coolant serves to preserve the mechanical integrity of the rollers.